As is well known, binoculars provide a magnified view of an image to each eye. The optics to provide such an image include various lenses and perhaps prisms. To focus the binoculars and compensate for the user's prescription or diopter, the binoculars are provided with ocular lenses at eyepieces one or both of which are adjustable for focusing. As is commonly found in binoculars, and as an example of the prior art, one of the eyepieces is adjustable to the user's particular diopter. Usually, the adjustable eyepiece is threadably coupled to a tube fixed against rotation to the binocular casing. In addition to the adjustability of the eyepiece (hereinafter referred to as the diopter eyepiece) a common adjustment may be provided to simultaneously move both eyepieces to focus the image based upon its distance from the observer. Hence, to operate the binoculars, the user views an object and manipulates the common adjustment until the object is in focus in the eyepiece which is not the diopter eyepiece. Thereafter, the diopter eyepiece is rotated relative to the tube causing the ocular lens to focus the image of the object. As adjusted, a magnified, binocular view of the object is provided. Thereafter, i.e. after the diopter eyepiece has been adjusted to focus the image depending upon the user's prescription, the common adjustment is used to focus the binoculars based upon the relative distance of the particular object to be viewed from the observer.
Of course, it is to be understood that some binoculars may provide for independent adjustment of both eyepieces. Further, for telescopes and monoculars a single eyepiece is provided which is adjustable to focus an image based upon not only the user's prescription but also the relative distance of the image from the observer.
A problem heretofore encountered with eyepieces which are adjustable (and particularly those which are adjusted relatively infrequently, such as a diopter eyepiece for binoculars) is that the diopter eyepiece may be inadvertently rotated, thereby rendering the binoculars out of focus. For example, the observer may inadvertently twiddle the diopter eyepiece or during walking or other movement the contact of the diopter eyepiece with clothing or the like may tend to rotate the eyepiece out of focus. For activities such as bird watching or hunting, an out of focus device presents problems. With reference to binoculars, should the eyepiece be out of focus, when the observer raises the binoculars and adjusts the common adjustment based upon the range of the bird, the image will be out of focus. Since the bird or big game may be visible for only a few seconds, the time necessary to readjust the diopter eyepiece may frustrate clear observation of the bird or beast.